Known oxide ferroelectrics which are employed as capacitors and transducers include, for example, Pb(Zr,Ti)O.sub.3 having the Perovskite structure and Bi-layer Perovskite compounds.
The term "Perovskite structure" used herein is intended to mean a crystal structure represented by the general formula, ABO.sub.3, wherein the BO.sub.6 oxygen octahedra with consists of six-coordinated octahedra of oxygen ions (O) about a given cation (B) undergo mutual corner sharings to form a three-dimensional fundamental structure in such a way that the resultant spaces are occupied with another type of cation (A). The above-indicated Pb(Zr,Ti)O.sub.3 represents a compound having the Perovskite structure wherein Pb corresponds to the A ion and the (Zr, Ti) corresponds to the B ion. In the above formula, the parentheses ( ) mean that at least one element selected from the elements included in the parentheses should be principally contained. It is a matter of course, however, that if a mixing ratio is defined, the elements being mixed are contained in such a compositional ratio.
Moreover, the term "layer Perovskite structure" is intended to mean a crystal structure wherein pseudo-Perovskite structures and intermediate layers (i.e. a layer intercalated between adjacent pseudo-Perovskite structures) are alternately located, as having such a structure as of pseudo-Perovskite structure/intermediate layer/pseudo-Perovskite structure/intermediate layer/ . . . , at a given period in one direction. The term "pseudo-Perovskite structure" means a structure which fundamentally consists of a Perovskite structure but whose elemental ratios are not exactly represented by ABO.sub.3 because the pseudo-Perovskite structure is invariably contiguous to the intermediate layers.
Further, the term "Bi-layer Perovskite structure" means such an intermediate layer as mentioned above wherein the cation in the intermediate layer consists of Bi. For instance, known compounds having a typical layer Perovskite structure include (Bi.sub.2 O.sub.2)(A.sub.n-1 B.sub.n O.sub.3n+1). In this instance, the intermediate layer consists of (Bi.sub.2 O.sub.2) and the pseudo-Perovskite structure consists of (A.sub.n-1 B.sub.n O.sub.3n+1). The suffix, n, means that corner-shared BO.sub.6 oxygen octahedral are stacked n times within the pseudo-Perovskite structure along the direction wherein the pseudo-Perovskite structures and intermediate layers are alternatively located.
FIG. 1(b) is a schematic projection view of a crystal structure, as viewed from the side of the layer, of a Bi-layer Perovskite compound of the formula wherein n=3, (Bi.sub.2 O.sub.2)(A.sub.2 B.sub.3 O.sub.10). In the figure, rectangles 11 each having a "cross" symbol therein, respectively, represent a BO.sub.6 octahedron formed by oxygen ions octahedrally coordinated about the B ion. These BO.sub.6 octahedra are three-dimensionally shared at the corners thereof to form a pseudo-Perovskite structure 18. The space between the adjacent BO.sub.6 oxygen octahedra 11 is occupied with an A ion 12. A region surrounded by a dotted line 15 indicates a Bi.sub.2 O.sub.2 intermediate layer isolating adjacent pseudo-Perovskite structures 18 from each other. This intermediate layer is constituted of Bi ions 13 and O ions 14.
A pure Perovskite structure has elemental ratios of A:B:O of 1:1:3. With the pseudo-Perovskite structure, the elemental ratios are n-1:n:3n+1, disenabling one to express the elemental composition as ABO.sub.3. As having set out hereinbefore, this is because the pseudo-Perovskite structures and the intermediate layers adjoin each other at a given period. As will be apparent from FIG. 1(b), the sites to be occupied by the A ions 12 are occupied by the Bi ions 13.
The compounds of the afore-indicated formula wherein n=2 are known including those compounds wherein A is an element selected from Pb, Ba and Sr, and B is at least one element selected from Nb and Ta. These compounds may be represented by Bi.sub.2 O.sub.2.AB.sub.2 O.sub.7 or ABi.sub.2 B.sub.2 O.sub.9.
As a compound wherein n=3, there is known a compound wherein A is Bi and B is Ti. In this case, the composition can be represented by Bi.sub.2 O.sub.2.Bi.sub.2 Ti.sub.3 O.sub.10 or Bi.sub.4 Ti.sub.3 O.sub.12.
Known compounds wherein n=4 include ones wherein A consists of an element selected from Sr, Ba and Pb, and Bi, and B is Ti. The composition of the compound can be represented by Bi.sub.2 O.sub.2.ABi.sub.2 Ti.sub.4 O.sub.13 or Bi.sub.4 Ti.sub.4 O.sub.15.
It will be noted that this kind of Bi-layer Perovskite compound is set forth, for example, in Meter. Res. Bull., 6(1971), pp. 1029.
Oxide ferroelectrics having the Perovskite structure represented by the formula, PbZr.sub.x Ti.sub.1-x O.sub.3, are in the form of a solid solution wherein the mixing ratio between PbTiO.sub.3 and PbZrO.sub.3 is x:1-x. When the mixing ratio is changed by changing x from 0 to 1, physical constants, such as spontaneous polarization, coercive field, dielectric constant and piezoelectric constant, can be, respectively, changed as desired, thereby enabling one to obtain desired values of ferroelectric characteristics, a dielectric characteristic, piezoelectric characteristics and the like. Especially, in the vicinity of x=0.54, the coercive field is minimized and the spontaneous polarization is maximized, so that such an oxide ferroelectric material exhibits the most excellent ferroelectric characteristics when used as a capacitor.
The Bi-layer Perovskite compounds represented by the formula, Bi.sub.2 O.sub.2.A.sub.n-1 B.sub.n O.sub.3n+1, exhibits great anisotropy of the crystal structure as mentioned above and thus, have great anisotropy with respect to the ferroelectric characteristics. For instance, with the compound represented by the formula, Bi.sub.2 O.sub.2.Bi.sub.2 Ti.sub.3 O.sub.10 or Bi.sub.4 Ti.sub.3 O.sub.12 wherein n is 3, A is Bi and B is Ti, the spontaneous polarizations along the directions parallel to and vertical to the layers are, respectively, 50 .mu.C/cm.sup.2 and 4 .mu.C/cm.sup.2 and differ from each other on the order of magnitude, thus showing remarkable anisotropy. The compounds exhibit great anisotropy with respect to the electromechanical coupling factor. The use of the compound, for example, as a transducer comprising an ultrasonic oscillator is advantageous in a small limitation on the width-to-thickness ratio.
Moreover, the compounds has the feature that the coercive field is small to allow easy polarization along the direction of stacking. In addition, the Bi-layer structure is unlikely to suffer degradation of spontaneous polarization resulting from the polarization hysteresis cycles. This is because the pseudo-Perovskite structure from which ferroelectric characteristics are derived is buffered with the Bi.sub.2 O.sub.2 intermediate layer at a molecular level, so that the mechanical strain in the crystals accompanied by the polarization hysteresis is absorbed.
On the other hand, however, with known Bi-layer Perovskite compounds shown in FIG. 1(b), the pseudo-Perovskite structures 18 are so arranged as to glide, through the intermediate layer 15 consisting of the Bi.sub.2 O.sub.2, by half a period within a plane parallel to the Bi.sub.2 O.sub.2 layer 15. Accordingly, the unit cell of the crystal lattices vertical to the Bi.sub.2 O.sub.2 layer 15 is, for example, as great as 2.6 nm for n=2, 3.3 for n=3, and 4.2 nm for n=4.
Such a long-range period structure not only takes a long time for the reaction at thermal equilibrium, but also presents the problem that lattice defects are liable to be introduced into the crystals. Further, the Bi.sub.2 O.sub.2 layer 15 restricts the size of the unit cell within a plane parallel to the layer. Eventually, the lattice constant is limited to a value as small as 0.54 nm to 0.55 nm (when defined as a pseudo-tetragonal system). Accordingly, the ion used as the B ion is limited to one having a small ionic radius such as Ti, Nb or Ta. In fact, there has not been obtained any compound, wherein part or all of the B ion is replaced by Zr, such as a solid solution, Pb(Zr, Ti)O.sub.3, consisting of PbTiO.sub.3 and PbZrO.sub.3.
If novel types of Bi-layer Perovskite ferroelectric compounds which comprise both Pb(Zr,Ti)O.sub.3 structures having good ferroelectric characteristics and Bi intermediate layers capable of imparting various characteristic features to the compounds are obtained, wide utility will be expected not only in the fields of their preparation and methods for making thin films, but also in applications thereof as capacitors, ultrasonic oscillators and the like.